The pattern of 5-methylcytosine residues in mammalian DNA has recently been found to be crucial to the control of genetic expression. Decreases in DNA 5-methylcytosine content are known to alter the level of differentiation of cells in culture. Thus changes in DNA 5-methylcytosine patterns may be critical to the process of carcinogenesis. Human tumor DNAs will be probed for DNA methylation pattern alterations in selective DNA sequences and genes. Since chemical carcinogens have been shown to decrease genomic 5-methylcytosine levels in BALB/3T3 cells, DNA from carcinogen-treated human epithelial cells will also be probed for changes in 5-methylcytosine patterns.